I've been doing some more tests with custom maps by creating them both in Explorer's Create A Ride section and on my desktop PC in Google My Maps for comparison. It looks like the game's Create A Ride uses the driving route only although there is an option to disable highways. When creating a map in Google My Maps on my computer, you can either do a map for driving, cycling or walking and each one will be different and use some different roads (GMM doesn't give you the option of choosing alternate routes like the standard Google Maps does) and Google My Maps doesn't have an option to disable/avoid highways.
It seems choosing a cycling route in GMM was perhaps causing the problem with missing sections, random white dots/plot points in the game and problems when jumping to the next location. I think this is because Explorer requires the use of continuous Google Street View imagery (not still images) and sometimes that's not available on certain roads or the cycling route goes down roads and paths where a car can't go and vice versa. Because Explorer generally works better with driving/car-based routes, it has problems with anything else.
This also causes a problem when creating custom routes in Google My Maps (I couldn't really create custom routes in Explorer's Create A Ride due to you only being able to input a start and end point to create directions) where you may want to use specific roads. If you create a driving route then you will be restricted to roads a car can use but you may want to go on a different road that's more suitable to cycling. You can't then switch to a cycling route and use those specific roads as it will alter the entire route.
As an example, I tried to create a custom route that closely followed the coast of Cornwall and Devon. I had Google Earth opened in one tab in my browser and Google My Maps in another, I wanted to be able to see exactly which roads had Street View as I assumed they would be usable in the game. When creating my route, I selected a driving route at first and dragged the little blue line where I wanted it to go, I had to make adjustments along the way by dragging the line and creating little white dots (plot points?) to make the route use specific roads. I constantly checked Google Earth to make sure I was only using Street View roads (I wanted to avoid as much "Jumping To Next Location" as possible and make the route smooth and continuous). I found that sometimes the line in My Maps wouldn't use specific roads I dragged it to (I don't know if that's because I ran out of plot points to adjust the line or not), I then checked Google Earth and went into Street View on some roads and found signs indicating they were cycle paths and didn't appear to be used by vehicles. So I changed the route style from driving to cycling so I could use those paths but then found this changed the entire route and that I had the opposite problem where the cycle route wouldn't use roads that only a car could use. It would also use paths/lanes that didn't have Street View.
So it seems that custom mapping is very difficult to do in My Maps and you're best using the driving route but then this will use highways as you can't disable them (unless of course you drag the line away from them). If you want the easiest method then create a route in Explorer itself with the Create A Ride feature as you can disable highways on that but you can't be as specific about where the route goes. There are also some problems when trying to input the start and end locations, I found a few instances of "No data for that location please try another" despite there being continuous street view data for where I wanted to place the pin or the locations I searched for.
Importing smaller sections/maps of an overall long route is better but I am not sure what is an ideal size for these sections or stages if you like and how customisable they can be. Another route I am interested in doing is The Great Tour https://thegreattour.co.uk/the-route/ which is a cycle route around the whole of the UK (excluding Ireland). I am not sure if those smaller sections would be better suited to the game or if it's even possible to create custom routes based on these. Perhaps I shall just have to create routes in Explorer based on each section using whatever route it creates and be happy with that.
It seems choosing a cycling route in GMM was perhaps causing the problem with missing sections, random white dots/plot points in the game and problems when jumping to the next location. I think this is because Explorer requires the use of continuous Google Street View imagery (not still images) and sometimes that's not available on certain roads or the cycling route goes down roads and paths where a car can't go and vice versa. Because Explorer generally works better with driving/car-based routes, it has problems with anything else.
This also causes a problem when creating custom routes in Google My Maps (I couldn't really create custom routes in Explorer's Create A Ride due to you only being able to input a start and end point to create directions) where you may want to use specific roads. If you create a driving route then you will be restricted to roads a car can use but you may want to go on a different road that's more suitable to cycling. You can't then switch to a cycling route and use those specific roads as it will alter the entire route.
As an example, I tried to create a custom route that closely followed the coast of Cornwall and Devon. I had Google Earth opened in one tab in my browser and Google My Maps in another, I wanted to be able to see exactly which roads had Street View as I assumed they would be usable in the game. When creating my route, I selected a driving route at first and dragged the little blue line where I wanted it to go, I had to make adjustments along the way by dragging the line and creating little white dots (plot points?) to make the route use specific roads. I constantly checked Google Earth to make sure I was only using Street View roads (I wanted to avoid as much "Jumping To Next Location" as possible and make the route smooth and continuous). I found that sometimes the line in My Maps wouldn't use specific roads I dragged it to (I don't know if that's because I ran out of plot points to adjust the line or not), I then checked Google Earth and went into Street View on some roads and found signs indicating they were cycle paths and didn't appear to be used by vehicles. So I changed the route style from driving to cycling so I could use those paths but then found this changed the entire route and that I had the opposite problem where the cycle route wouldn't use roads that only a car could use. It would also use paths/lanes that didn't have Street View.
So it seems that custom mapping is very difficult to do in My Maps and you're best using the driving route but then this will use highways as you can't disable them (unless of course you drag the line away from them). If you want the easiest method then create a route in Explorer itself with the Create A Ride feature as you can disable highways on that but you can't be as specific about where the route goes. There are also some problems when trying to input the start and end locations, I found a few instances of "No data for that location please try another" despite there being continuous street view data for where I wanted to place the pin or the locations I searched for.
Importing smaller sections/maps of an overall long route is better but I am not sure what is an ideal size for these sections or stages if you like and how customisable they can be. Another route I am interested in doing is The Great Tour https://thegreattour.co.uk/the-route/ which is a cycle route around the whole of the UK (excluding Ireland). I am not sure if those smaller sections would be better suited to the game or if it's even possible to create custom routes based on these. Perhaps I shall just have to create routes in Explorer based on each section using whatever route it creates and be happy with that.